In the drilling of deep wells the drilling bit may be driven by a positive-displacement type motor located down the hole towards the lower end of a drilling string composed of a number of sections through which liquid mud is fed under pressure to drive the motor, scavenge the hole around the bit and carry away cuttings and the like upwardly to the surface through the annular space between the drilling string and the surrounding wall of the hole.
In order to generate the necessary torque in the motor to drive the drilling bit, high liquid mud pressures are required and the motor is exposed to onerous operating conditions by virtue of such pressures and the nature of the driving liquid.
One particularly suitable type of positive displacement motor comprises a rotor rotatable in a housing with the annular space between the rotor and the housing divided into at least two chambers by longitudinally extending separators secured to the wall of the housing with a plurality of flexible blades attached to the rotor so that they swing out and engage the housing wall in fluid sealing relationship under the pressure of the liquid mud admitted to the chambers and fold inwardly when they engage the separator strips. The number of blades corresponds to or is a multiple of the number of chambers and as they pass a separator they are exposed to an inlet for the liquid mud and as they reach a separator the liquid mud escapes through an outlet, thus relieving the pressure on the blades and allowing them to fold inwardly.
Examples of such motors are disclosed in my U.S. Pat. Nos. 2,852,230 dated Sept. 16, 1956; 3,076,514 dated Feb. 5, 1963; and 3,594,106 dated July 20, 1971. Such motors disclosed in the prior art referred to are undoubtedly useful in the down-hole drilling of wells but it will be appreciated that the blades are exposed to extremely high liquid mud pressures in order to generate the necessary driving torque. Because of such high fluid pressures and the nature of the driving fluid, the motors operate under extremely onerous conditions and any improvement serving to increase the life of the motor under field operating conditions is important.
It is essential to maintain a good fluid-tight seal between the moving longitudinal edges of the blades and the wall of the housing so that fluid pressure should not be lost and the efficiency of the motor reduced. At the same time this is the area at which wear is most likely to occur due to friction and rubbing and entrained particles in the liquid mud.
In my above referred to United States patents the blades are mounted in longitudinally extending slots in the rotor and in U.S. Pat. Nos. 2,852,230 and 3,076,514, the inner edges of the blades are backed against and fully supported by a wall of the slot which limits the outward swing of the blades when exposed to fluid pressure. As is recognized in my U.S. Pat. No. 3,594,106, this results in the outer unsupported part of the blade tending to roll back about the outer inclined edge of the slot when exposed to fluid pressure thereby increasing the load on the blade and the tendency for the blade to lock against the wall of the housing. In order to reduce this problem, my U.S. Pat. No. 3,594,106 discloses a space between the lower corner of the blade and the lower corner of the slot in which it is fitted together with small projections on the blade in the form of small cylindrical buttons of a soft resilient elastomer which serve to keep the blade in contact with the supporting wall of the slot but which yield to a limited degree under pressure. Thus, instead of the inner portions of the blade being fully supported by the wall of the slot as in my two earlier patents referred to above when being driven under pressure, the inner portions of the blade are yieldingly supported by the wall of the slot, the yield being a function of the applied pressure and cooperating with the stop means provided by the outer inclined edge of the slot in the sense of tending to pivot the blade about this inclined edge in the direction opposite to that in which the pressure applied to the outer unsupported part of the blade tends to turn the blade. This structure materially reduces the wear of the blades and enhances the life of the motor under field operating conditions. In each of the above-mentioned patents the discharge ports are located immediately before the leading ends of the separators and the blade strikes the separator with great force, thus producing considerable wear on both the blade and separator.